The present invention relates in general to semiconductor switching devices in a power module for an inverter bridge, and, more specifically, to inverter drive systems for electrified vehicles using discrete switching devices in a power module with structures for enhancing a common source inductance.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque. A typical electric drive system may include a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC linking capacitor. An inverter is connected between the main buses and a traction motor in order to convert the DC bus power to an AC voltage that is coupled to the windings of the motor to propel the vehicle.
The inverter includes transistor switching devices (such as insulated gate bipolar transistors, IGBTs) connected in a bridge configuration with a plurality of phase legs. A typical configuration includes a three-phase motor driven by an inverter with three phase legs. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor. The inverter may pulse-width modulate the DC link voltage in order to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. Pulse Width Modulation (PWM) control signals applied to the gates of the IGBTs turn them on and off as necessary so that the resulting current matches a desired current.
Common source inductance refers to an inductance shared by the main power loop (i.e., the drain-to-source or collector-to-emitter power output of the transistor) and the gate driver loop (i.e., gate-to-source or gate-to-emitter) in a power switching transistor. The common source inductance carries both the device output current (e.g., drain-to-source or collector-to-emitter current) and the gate charging/discharging current. A current in the output (i.e., the power loop) portion of the common source inductance modifies the gate voltage in a manner that reinforces (e.g., speeds up) the switching performance. For a switching bridge, the reduced switching time may be desirable since it may have an associated reduction in the energy consumed (i.e., lost) during the switching transition. Modeling of circuit voltages, currents, and switching operation can determine an optimal magnitude for the common source inductance.
The magnitude of the gate loop inductance and/or the power loop inductance and the degree of mutual coupling between them can be manipulated (e.g., enhanced) by selecting an appropriate layout and/or including added overlapping coils in PCB traces forming conductive paths to the transistor gates or emitters in order to obtain a desired common source inductance LCSI. Examples are shown in U.S. patent application publications US2018/0152113A1, US2018/0159440A1, and US2018/0123478A1, and U.S. Pat. No. 9,994,110, each of which is incorporated herein by reference in its entirety.
The power modules typically generate a large amount of heat, so they are often attached to a coldplate (i.e., heatsink) for better thermal performance. Preferred materials for the coldplate include electrically conductive materials, such as aluminum or copper. When the power current flows through the power module, the time-varying magnetic flux of the power loop induces Eddy currents in the conductive coldplate. The Eddy currents create a magnetic field that opposes the original magnetic field from the power loop. The total magnetic flux is reduced, which lowers the effective inductances of the power loop. Consequently, the common source inductance can also be reduced by the Eddy currents, making it difficult to enhance the common source inductance as desired.